A. Field of the Invention
The present invention relates to a semiconductor device and a method for producing a semiconductor device.
B. Description of the Related Art
A power conversion apparatus, such as a converter-inverter system essential for the control of a rotary motor or a servo-motor, has been known. In order to improve the efficiency of the power conversion apparatus and to reduce the power consumption thereof, there is a strong demand for a technique capable of reducing the loss of a semiconductor device, such as a power diode or an insulated gate bipolar transistor (IGBT) provided in the power conversion apparatus.
As one of the methods for meeting the demand for the technique capable of reducing the loss, for a diode or an IGBT, a field stop (FS) layer structure has been known in which a drift layer, which is a thick layer with the highest resistance, among semiconductor layers forming an element structure is thinned to reduce a voltage drop due to an on-current, thereby reducing on-loss. In the FS layer structure, an FS layer which has a higher impurity concentration than the drift layer and has the same conduction type as the drift layer is provided at a position that is away from the breakdown voltage main junction of the drift layer in the drift layer. The provision of the FS layer makes it possible to suppress the spreading of a depletion layer which is spread from the breakdown voltage main junction in the high-resistance drift layer when the device is turned off. Therefore, it is possible to prevent punch-through even when the drift layer is thin.
In the manufacture (production) of the power device, a wafer (hereinafter, referred to as an FZ wafer) which is cut out from an ingot produced by a floating zone (FZ) method is used in order to reduce costs. The FZ wafer with a thickness of 600 μm or more is put into a manufacturing process in order to reduce the breaking of the wafer. Finally, the FZ wafer is ground to a thickness required for the designed breakdown voltage during the manufacturing process in order to reduce on-loss. In particular, in a MOS (metal-oxide film-semiconductor) device, such as an IGBT, after a MOS gate structure, a circumferential junction edge termination structure, and a metal electrode film are formed on the front surface of the FZ wafer, a grinding process for thinning the FZ wafer is performed for the rear surface of the FZ wafer. Then, after the rear surface of the FZ wafer is ground to reduce the thickness of the wafer, an FS layer or a collector layer is formed on the ground rear surface of the FZ wafer. Therefore, in the method according to the related art, there are restrictions that the FS layer is formed under the conditions that have no adverse effect on the semiconductor function layers provided on the front surface side of the FZ wafer. Therefore, it is not easy to form the FS layer. In general, the FS layer is formed with, for example, an n-type impurity element with a large diffusion coefficient. In some cases, in addition to an FZ wafer which is made of polysilicon with high crystal purity, an FZ wafer made of a CZ wafer or a CZ wafer with high resistivity is used.
In recent years, a method has been developed which forms the FS layer using a process of generating donors using proton irradiation. In the method for forming the FS layer using proton irradiation, a heat treatment is performed to recover the crystal defects which are generated in an FZ bulk wafer by irradiation with proton ions (H+) and protons in the vicinity of the average range Rp of protons in the FZ bulk wafer are changed into donors to form an n-type region with high concentration.
In some cases, when the n-type region with high concentration is formed by proton irradiation, the mobility of electrons/holes is reduced at the irradiation position of the proton, which is described in the patent literature (for example, see the following Patent Literature 1). In addition, proton irradiation conditions for forming a blocking zone (FS layer) and the preferred heat treatment conditions after proton irradiation have been proposed when the n-type region with high concentration is formed by proton irradiation (for example, see the following Patent Literature 3 to the following Patent Literature 7). Unlike other ions, protons are combined with the crystal defects in the semiconductor layer to recover carrier concentration. As the concentration of the crystal defects generated in the semiconductor layer during proton irradiation increases, higher carrier concentration is obtained, which is described in the patent literature (for example, see the following Patent Literature 2).
The following Patent Literature 1 discloses a region in which the mobility of electrons/holes is reduced due to proton irradiation. Specifically, it has been reported that the mobility of carriers is reduced by a high-concentration crystal defect layer which is formed in the vicinity of the rear surface of the wafer by proton irradiation. The following Patent Literature 2 discloses a structure in which, when the crystal defects generated by proton irradiation are recovered by a heat treatment, the remaining amount of crystal defects is so large that a donor layer is not removed by protons. The general impurity atoms, such as phosphorous (P) atoms or arsenic (As) atoms which are present at the lattice positions of silicon (Si), exchange an outermost electron. In contrast, in the above description, a donor (hereinafter, referred to as a hydrogen-related donor) caused by hydrogen (H) supplies an electron from a composite defect of a plurality of lattice defects (for example, divacancies) which are formed in silicon by proton irradiation and the radiated hydrogen atom.